Using excellent properties such as high specific strength and high corrosion resistance, many titanium alloy products have been used as, for example, aircraft construction materials. Meanwhile, for use as consumer products, the titanium alloy products have been widely used as muffler members for automobiles/motorcycles, glasses frames, sports tools (such as golf club faces, parts for spikes, and metal bats), and the like.
As one of defects of the titanium alloy, there is given that the Young's modulus is lower than the Young's modulus of a steel material and the like. With a low Young's modulus, there is a problem in that elastic deformation likely occurs (rigidity is low) in the case where the titanium alloy is used as structural materials and parts. Further, in the case where the titanium alloy is used as a golf club face, for example, since the face is likely to deflect, a coefficient of restitution is apt to be large, and there is a problem in that it is difficult to satisfy a coefficient-of-restitution regulation.
In this case, in the case where the shape of a product is an elliptic or rectangular sheet, it is already known that a high Young's modulus in the short-side direction makes the deflection less likely to occur, and is effective as means to increase the rigidity of the sheet. In order to obtain such a state, Patent Literature 1 discloses technology for increasing the strength and the Young's modulus in the sheet-width direction by performing unidirectional hot-rolling on an α+β titanium alloy and controlling the texture. In this technology, an α+β alloy is subjected to unidirectional hot-rolling under specific conditions to develop a hot-rolling texture that is called transverse-texture in which a basal plane of a titanium α phase is strongly orientated in the sheet-width direction, and thus, the strength and the Young's modulus in the sheet-width direction are increased. In this case, it becomes possible to make it difficult to deflect an elliptic or rectangular sheet-like product by setting the sheet-width direction of the hot-rolled sheet to the short-side of the sheet-like product.
In this manner, for use as golf club faces, for example, application of α+β titanium alloys each having a high Young's modulus is the mainstream under the environment in which the coefficient-of-restitution regulation has become strict. With the use of an α+β titanium alloy having a high Young's modulus, the coefficient of restitution hardly increases even if the thickness of the face decreases, and the degree of freedom of the sheet thickness for clearing the coefficient-of-restitution regulation increases compared to a β titanium alloy having a low Young's modulus. Further, there are many advantages in that, compared to the β titanium alloy, the α+β titanium alloy is smaller in specific gravity so that the volume of a club head can be increased with the same mass, and is also smaller in content of expensive alloying elements so that the cost of materials is low. As the α+β titanium alloy, Ti-6% Al-4% V is typically used, and in addition, examples of the α+β titanium alloy also include Ti-5% Al-1% Fe, Ti-4.5% Al-3% V-2% Fe-2% Mo, Ti-4.5% Al-2% Mo-1.6% V-0.5Fe-0.3% Si-0.03% C, Ti-6% Al-6% V-2% Sn, Ti-6% Al-2% Sn-4% Zr-6% Mo, and Ti-8% Al-1% Mo-1% V, Ti-6% Al-1% Fe.
Moreover, for use as golf club faces, it is desirable that a thin-sheet material or the like in which molding processability at the time of processing a face is low and freedom in meeting the coefficient-of-restitution regulation with shape control is low have a Young's modulus in one direction in the plane of the sheet of more than or equal to 135 GPa and tensile strength of more than or equal to 1100 MPa. In this case, it is desirable that the Young's modulus satisfy the above value in order to clear the coefficient-of-restitution regulation, and it is desirable that the tensile strength and ductility satisfy the above value in order to obtain satisfactory fatigue properties. However, in general, processability of an α+β alloy is not satisfactory, and even if the sheet thickness is decreased, there are few alloys which have excellent fatigue properties, high strength and a high Young's modulus that satisfy the coefficient-of-restitution regulation, and satisfactory hot workability. Further, high values in fatigue properties and/or impact toughness have not been achieved yet, which influence durability of golf club faces. That is, no technology has been disclosed yet which relates to a titanium alloy having a high Young's modulus and high fatigue strength and/or impact toughness.
Further, oxygen contained in a titanium alloy is known as an element that is likely to segregate at the time of manufacturing an ingot, and, although a titanium alloy containing a large amount of oxygen has high strength, there is a problem in that different concentrations caused strength variation within an ingot. In addition, there is also a problem in that when oxygen is contained excessively, the ductility decreases considerably.
For example, Ti-6% Al-4% V alloy, which is a most general-purpose α+β alloy, has sufficient strength and Young's modulus, and is already used widely as structural members such as aircraft construction material parts. However, this alloy has problems in that: the alloy contains 6% of Al, which has a high solid-solution-strengthening ability and increases deformation resistance at the time of hot working, and the hot workability is not satisfactory; the alloy contains 4% of V, which is an expensive β stabilizer element, and the cost of the material is relatively high; and the alloy is strengthened by solid-solution-strengthening owing to O, as will be described later, and hence, the fatigue strength is not sufficient.
Patent Literature 2 discloses a low-cost alloy having high specific strength in the same manner as Ti-6% Al-4% V alloy. This is an α+β alloy aiming at gaining high specific strength and low cost by adding a large amount of Al which is an a stabilizer element having low specific gravity. However, this alloy contains 5.5 to 7% of Al, and has a disadvantage in that it is difficult to be subjected to hot working. In order to lower the processing cost for the face material, a supply of a sheet product that can be processed into a face shape only through easy press forming and polishing steps is desired. In manufacturing a hot-rolled sheet of the alloy, however, the range of the appropriate hot-rolling temperature is small due to high hot deformation resistance, and even if the temperature is slightly lower than the range, edge cracking easily occurs to cause a problem of a decrease in production yield. Further, strength variation due to segregation of oxygen is also present.
Patent Literature 3 discloses a golf club head including a high strength and low resilience titanium alloy face. It defines the contents of Al and Fe in the titanium alloy for forming the face, and describes that therefore a high Young's modulus and tensile strength can be obtained. Although Patent Literature 3 does not describe a specific method of manufacturing the alloy, the manufacturing method is limited to some extent in order to obtain tensile strength of 1200 to 1600 MPa as recited in Claims in the alloy composition containing Al, Fe, and the balance of inevitable impurities as shown in Claims. That is, such strength cannot be obtained in the case of as-hot worked such as hot-rolling and forging, or in the case of performing annealing treatment after hot working or cold working. In addition, a product in this strength range cannot be obtained also in the case of subjecting a hot- or cold-worked product to aging heat treatment, but may be obtained only in a state of as-cold worked which is processed up to a high processing degree. However, when the as-cold worked material is used for a golf club face, high strength can be obtained but fatigue properties decrease remarkably, therefore, once a fatigue crack occurs on the face, the propagation of the fatigue crack cannot be stopped. Thus, there is a problem in that excellent fatigue properties necessary for golf club faces cannot be ensured.
Patent Literature 4 discloses a titanium alloy sheet for a face in which fatigue properties of a heat-affected zone in a golf club head including a weld zone are high, and in which a Young's modulus and strength are adjustable by heat treatment. It is characterized in that addition of appropriate amounts of Al, Fe, O, and N adjusts the strength and enhances the fatigue properties of the heat-affected zone, and control on heat treatment conditions such as aging strengthening heat treatment controls the Young's modulus. However, after Patent Literature 4 was filed, the coefficient-of-restitution regulation was introduced and only alloys with a high Young's modulus have been demanded. With the sheet product manufactured with the alloy composition under the manufacturing conditions recited in Claims of Patent Literature 4, there is the problem in that sometimes a high Young's modulus which satisfies the coefficient-of-restitution regulation cannot be obtained. Further, strength variation due to segregation of oxygen similar to that written in Patent Literature 2 is also present.
Patent Literature 5 discloses technology for enhancing coil handleability during cold working, for example, the technology includes subjecting a titanium alloy containing Al, Fe, O, and N to unidirectional hot-rolling and developing the above-mentioned texture called transverse-texture, to thereby suppress occurrence of fracture in the sheet during coil winding. With the development of the transverse-texture, even if edge cracking to be the starting point of the sheet fracture occurs, the crack propagates obliquely and the length of the crack increases. However, no consideration is given to solve the technical problems of a high Young's modulus, high fatigue properties, strength ununiformity, and the like.
Moreover, Patent Literature 6 discloses an α+β titanium alloy containing Al, Fe, and Si, and discloses that the α+β titanium alloy has the same fatigue strength and creep resistance as a conventional Al—Fe-based titanium alloy. However, no consideration is given to the technical problems on the high Young's modulus, strength ununiformity, and the like.
Patent Literature 7 discloses a method of manufacturing an α+β titanium alloy, the method including: heating a titanium alloy containing Al, Fe, Si, and O, and further containing selectively Mo and V to a temperature higher than or equal to a β transus temperature, starting hot-rolling at lower than or equal to the β transus point, and performing hot-rolling mainly at higher than or equal to 900° C. Although it is written that the thus manufactured titanium alloy can decrease surface flaws that occur on the surface of the hot-rolled sheet, there is no disclosure of technology related to a titanium alloy having a high Young's modulus, high strength, excellent fatigue properties, and uniform strength.
Patent Literature 8 discloses a near-β α+β alloy to which Si is added and which is excellent in fracture toughness, and a manufacturing method thereof. However, the toughness is evaluated with fracture toughness values, not with a property related to impact toughness including deformation under a high rate of strain determined by a Charpy test or the like. Further, the microstructure is limited to an acicular structure.
Here, the fracture toughness is generally a material property indicating the ability of a material to resist crack propagation under a relatively low rate of strain, and is generally evaluated by performing a fracture toughness test. For example, the evaluation may be performed using Unloading Elastic Compliance Method shown in Non-Patent Literature 1. On the other hand, the impact toughness is a property indicating the ability of a material to resist fracture under a high rate of strain, and can be evaluated easily by using absorbed energy of the Charpy impact test. Since golf clubs and automobile parts are exposed to deformation at a high rate, it is desired that the impact toughness be high.
That is, no technology has been disclosed yet, which relates to an α+β titanium alloy satisfying simultaneously a high Young's modulus, high strength, excellent fatigue properties, and excellent impact toughness, which are required for high-grade golf club faces or some automobile parts. Further, technology taking into consideration strength variation due to segregation of oxygen within an ingot has also not been disclosed yet.